Salt Domes Used as Underground Storage Tanks
The petro-physical properties of salt guarantee the natural tightness of the rock salt caverns and make an additional lining, such as required for storage caverns in disused mine workings, unnecessary.
The use of salt caverns as underground gas storage facilities depends on the occurrence of huge underground salt deposits at accessible depths. The salt domes in northern and central Germany are particularly suitable for this. Large cavities for storing gas can be created in the rock salt, which is mainly located in socalled salt stocks only a few hundred metres under the earth's surface, and can easily be tapped through borings.
Up to 100 million m3 of storage volume
These boreholes are then used to inject water into the deeper rock salt layers and to pump the dissolved salt to the earth's surface as brine. The cavern is in most cases cylindrical in shape. Depending on the size, the heights of such caverns vary between 100 and over 500 metres and the volumes of gas stored between 40 and 100 million m3 per cavern. The cavities formed resemble underground tanks and the borehole is the only way of injecting gas into or withdrawing gas from them.
Compressors are used to inject the gas into the caverns and store it there under pressure. This way, it can be withdrawn quickly at any time for peak-shaving.
Cushion gas provides the necessary pressure
The gas held in a gas storage facility is always divided into cushion and working gas. The cushion gas is the volume of gas that is necessary to ensure the minimum storage pressure necessary for optimal gas injection and withdrawal.
In storage caverns, the cushion gas is also necessary to ensure stability. The proportion of cushion gas is roughly one third to a half of the maximum storage volume and remains permanently in the storage facility.
Working gas - a flexible quantity
Working gas is the gas volume which can be stored or withdrawn at any time in addition to the cushion gas.